This invention relates to the purification of polymer solutions and more particularly to elimination of the hazy, cloudy or milky appearance often observed in organic solutions of polymers prepared by anionic polymerization.
A common occurrence when redissolving in an organic medium polymers synthesized with an anionic initiator is a cloudy, milky or hazy appearance indicative of a residue of the polymerization reaction. Usually, the polymer solution directly resulting from the anionic polymerization will be clear and the cloudy condition will appear when the polymer is isolated and redissolved. Sometimes the hazy appearance also occurs upon aging of a polymer solution directly resulting from the synthesis. Purification is exceedingly difficult to achieve by conventional filtration practiced alone or with filtration aids. For example, the impurities have been found resistant to treatment with adsorbents such as alumina, talc, silica gel, various forms of alumina, clays and carbon black. The impurities in such solutions have also been found resistant to conventional salting out techniques using materials such as sodium chloride, sodium acetate, sodium carbonate, sodium bicarbonate and calcium carbonate.
Removal of such impurities is important for many commercial uses of the polymers and organic diluent solutions of such polymers, for example when the polymers are intended for use as clear top coats for laminates and other substrates, or in adhesives. In this specification the term "impurity" or similar term means an undesired material in an organic diluent solution of such polymers observable as a cloudy, milky or hazy condition, and "purification" means removal of such material.
The problem of purifying polymers as in the present invention should be distinguished from efforts to prevent or reduce the formation of color and gel in polymer solutions. In U.S. Pat. No. 4,415,695 to S. B. Sarkar a method is described for preparing essentially color-free, stabilized polymers by the addition of boric acid to organic solutions of polymers resulting from anionically initiated syntheses, preferably with the reagent for terminating the polymer precursor, but in any event prior to the addition of polymer stabilizing agents. The boric acid is used in amounts ranging from 0.02 to 1.0 parts by weight per 100 parts by weight of the polymer, preferably 0.1 to 0.5 parts by weight on 100 parts of polymer. Following the addition of the boric acid and the stablizing agent, the polymer is recovered from the solution by coagulation, vacuum drying, or other concentration technique. By such treatment essentially colorless polymers are prepared as compared with slightly yellow or deep yellow untreated polymers. By visual examination of polymer samples which had been oven aged for 6 days at 212.degree. F., it was also determined that the boric acid treatment prevented the formation of polymer gel.
It will be apparent that purification in the sense of the present invention is not described or suggested in the Sarkar patent. In Sarkar, no effort was made to remove any material from the polymer solutions following the boric acid treatment. The boric acid treatment is entirely for the purpose of preventing color and gel formation, and the patent provides no indication of impurities in untreated polymer solutions in the sense of cloudy or hazy material, the removal of which is the objective of the present invention.